This invention relates to surgically implantable knee joint prostheses and more particularly to improvements in tibial components of such devices.
Knee arthroplasty is becoming more common to partially or totally replace knee joints which have been damaged due to trauma or disease. In some cases, only one compartment of the knee joint need be replaced while in other cases, both compartments of the femoral and tibial articulating surfaces must be replaced. Based upon the condition of the ligaments and tendons surrounding the joint, the surgeon selects a knee joint prosthesis which provides the necessary degree of stability to the joint. Implantation of such prostheses involves accurately shaping the ends of the bones to receive each component of the prosthesis in proper alignment and thereafter insuring that each component is properly sized to permit the repaired joint to function normally as best as is possible.
Prosthesis manufacturers provide a range of trial components to assist the surgeon in selecting appropriately sized components. For the tibial components, the surgeon is provided with a trial tibial base which is placed on the prepared tibial plateau and, for example, a set of slide-in plastic trial tibial base inserts (containing an articulating surface for femoral condyles) in various sizes which correspond to permanently implantable tibial prostheses. The trial base is notched to receive the oppositely notched underside of the trial inserts so that the surgeon can slide in a trial insert and test the joint for proper flexion, etc. The process may be repeated a number of times until a trial insert is found which is properly sized for the patient's knee joint. The trial base and trial insert are then removed and a permanent implant corresponding to the size of the trial tibial insert/base combination is then fixed to the tibial plateau. See, for example, Dow Corning Wright Corporation (Arlington, Tenn.) Brochure No. L095-0101 (12 pp., 1983) entitled "Whiteside ORTHOLOC (.TM.) Total Knee System" which shows one such trial base and trial insert design. The inserts freely slide in and out of the tibial base.
Other removable tibial base insert prostheses have been taught and used in the past, some to eliminate the need for a separate trial base. U.S. Pat. No. 4,016,606 to Murray, et al. (issued 4/12/1977) teaches a planar tibial base insert which is affixed within an implantable tibial base by sliding the tapered notched lower part of the insert into a tapered oppositely notched area in the tibial base and then locking the insert into the tibial base by means of a pin passing through the metal base and the insert. This has the disadvantage of requiring a separate pin to lock the insert in place although the patent does suggest that one might be able to omit the locking pin. Another example of slide-in tibial insert (one for each condyle) is shown as a "meniscal bearing" in DePuy, Inc (Warsaw Ind.) Brochure No. 7.5 M1084 0601-71 (Rev. 1) entitled "New Jersey Tricompartmental Total Knee System With POROCOAT (.RTM.), Surgical Procedure" (22 pp., 1984).
U.S. Pat. No. 4,207,627 to Cloutier (issued 6/17/1980) teaches the use of drop-in tibial inserts, one for each condyle. Each insert is placed in a recess in the implantable tibial base and the front of each insert is grooved to mate with an oppositely-grooved portion of the tibial base. The back portion of the insert is not grooved and simply butts against the wall of the recess. The back portion of the insert is therefore not locked in place. The removable inserts enable a surgeon "to obviate any misalignment, natural or not," although in doing so, the surgeon would have to remove the insert laterally where several ligaments are located rather than anteriorly.
In U.S. Pat. No. 4,257,129 (issued 3/24/1981), Volz teaches a replaceable articulation member tibial component which employs a central vertical pin and a two-prong horizontal clip which locks the articulation member in place after it is slid onto the tibial base up to a raised stop means. One object is to provide an articulation member which can easily be replaced in the event that the articulating surface of the removable member becomes worn. Use of a pin and a clip adds additional complexity to the manufacture and insertion/removal of the member. The Volz patent also suggests a tibial base and removable articulation member having raised dovetail bosses to retain the insert within the base, but does not tell how--other than by a "snug sliding fit" or interference fit--that removable member is to be locked into place to prevent it from sliding in the direction opposite (anteriorly) the raised tab in the base used to stop the member upon insertion.
A more recent removable tibial base insert prosthesis which can be removed anteriorly from the tibial base and which locks in place without a need for the clips and/or pins noted above is described in Zimmer, Inc. (Warsaw, Ind.) Brochure No. 84-038-5780-0352/15MA entitled "The MILLER/GALANTE Porous TIVANIUM (.RTM.) Total Knee" (18 pp., 1984). The tibial base is permanently implantable while various sized tibial articulating surface inserts can be fixed and removed using an inserting and a removing instrument. The tibial insert is slid over and secured to a central, approximately dove-tailed, eminence on the tibial base and the outside periphery of the base drops within a raised peripheral rim on the tibial base. Because the only point of actual attachment to the tibial base is at the central eminence, the tibial insert may become stressed and possibly lift away from the tibial base at one side or another in response to lateral movement of the knee joint since there is no locking attachment to the sides of the tibial base. While the Miller/Galante knee is not offered with posterior stabilized knee components, if such were available, this stressing and lifting of the tibial insert becomes more probable. If a posterior-stabilized knee prosthesis with a raised central post on the tibial insert is used with this type of removable tibial insert design, the post is designed to provide stability to the knee by fitting within and being constrained by an intercondylar recess in the femoral component of the total knee prosthesis. Lateral movement of the knee may cause the post to press against the walls of the recess and result in stress and possible lifting of the insert from the tibial base. In addition to stressing the tibial insert, when a posterior-stabilized knee prosthesis is used, the knee joint itself might possibly feel somewhat loose if the insert should bend and lift when subjected to a lateral motion since a great deal of force is exerted on the knee joint during articulation.